The present invention relates to steering systems for automotive vehicles. More specifically, the present invention relates to steering systems having linear independent actuators having an interlocking mechanism.
Conventional mechanically coupled steering systems use a single actuation rack to steer both steerable wheels. As an operator turns a steering wheel, mechanical couplings between the steering wheel and the actuation rack, as well as between the actuation rack and each steerable wheel, results in the steerable wheels turning together in tandem.
It is sometimes desirable, however, for a pair of steerable wheels to turn at different rates and angles. The development of steer-by-wire systems accomplishes this by providing a steering motor for each steerable wheel on a vehicle that allows for the turning of each steerable wheel at a different rate and angle. Further, development of steer-by-wire systems allows for the elimination of mechanical couplings between the operator controlled steering wheel and the steerable wheels, as well as the elimination of the need to have the steerable wheels of a vehicle mechanically coupled together. Such modifications reduced the cost and weight of the vehicle.
These systems, however, lack a fail-safe feature. Such a feature is required in situations, for example, where one steering motor becomes inoperable. The steerable wheel that was steered by the inoperable steering motor becomes either uncontrollable or becomes locked in position. This can create a safety hazard. Attempts have been made to create a mechanical coupling between the steerable wheels to provide a backup safety feature. For example, if a steering motor should fail, the backup safety feature allows the operable steering motor to steer both steerable wheels. Several problems exist, however, with such mechanical couplings.
In one example, a relatively small amount of play exists between the elements of the mechanical couplings and the two steerable wheels that are coupled together. Such an arrangement restricts movement of each steerable wheel relative to each other. Further, such traditional mechanical couplings require that the steerable wheels remain mechanically coupled together at all times. The mechanical couplings in this configuration tend to limit or even eliminate the ability to steer each steerable wheel separately.
In another example, a relatively large amount of play has been designed to exist between the elements of the mechanical couplings and the two steerable wheels that are coupled together. In such a system, if one steering motor becomes inoperable, the amount of play designed into the mechanical couplings results in lost motion between the steerable wheel with the operable steering motor and the steerable wheel with the inoperable steering motor. In such lost motion mechanical coupling systems, the mechanically coupled steerable wheel with an inoperable steering motor does not respond in tandem with the steerable wheel with an operable steering motor. This situation can create unfavorable and unsafe driving conditions for the operator as the steering input to the steering wheel may not result in the expected behavior from the steerable wheels.
In one aspect of the invention, a steering system is provided. The system includes a housing with a first end and a second end, a first steering motor is operatively connected to the housing, and a second steering motor is operatively connected to the housing. A first gear set is operatively connected to the first steering motor, and a second gear set is operatively connected to the second steering motor. A first screw device is operatively connected to the first gear set, and a second screw device is operatively connected to the second gear set. A first rack is operatively connected to the first gear set and is slidably connected to the first end of the housing. A second rack is operatively connected to the second gear set and is slidably connected to the second end of the housing. A differential operatively connected to the housing and capable of mechanically coupling the first steering motor and the second steering motor together is also provided.
Coupling the first steering motor and the second steering motor together through a differential forms a mechanically coupled steering motor unit and allows the mechanically coupled steering motor unit to move both the first rack and the second rack together with minimal lost motion between the mechanically coupled steering motor unit, the first rack, and the second rack.
A further aspect of the invention is embodied in a method for steering a vehicle. The method includes providing a steering system having a housing with a first end and a second end. A first steering motor is operatively connected to the housing. A first rack is operatively connected to the first steering motor and the first rack is slidably connected to the first end of the housing. A second steering motor is operatively connected to the housing. A second rack is operatively connected to the second steering motor and the second rack is slidably connected to the second end of the housing. A differential is operatively connected to the housing and is capable of selectively mechanically coupling the first steering motor and the second steering motor together.
Once a failure of one of the first steering motor and the second steering motor is detected, the operable steering motor and the inoperable steering motor are coupled together through the differential to form a mechanically coupled steering motor unit. The mechanically coupled steering motor unit is utilized to move the first rack and the second rack together with minimal lost motion between the mechanically coupled steering motor unit, the first rack and the second rack. Because there is minimal lost motion an operator may maintain steering control in the event one of the first steering motor and the second steering motor fails.
In another aspect of the invention, a method of torque sharing for a steering system is provided. The system includes a housing with an outer cornering end and an inner cornering end. A outer cornering steering motor is operatively connected to the housing. A outer cornering rack is operatively connected to the outer cornering steering motor and the outer cornering rack is slidably connected to the outer cornering end of the housing. An inner cornering steering motor operatively connected to the housing. An inner cornering rack is operatively connected to the inner cornering steering motor and the inner cornering rack is slidably connected to the inner cornering end of the housing. A differential capable of selectively mechanically coupling the first steering motor and the second steering motor together to form a mechanically coupled steering motor unit is also provided.
The method involves detecting an outer cornering steering load on the outer cornering rack and detecting an inner cornering steering load on the inner cornering rack. The steering loads on the outer cornering rack and the inner cornering rack are compared to determine whether the outer cornering rack has a greater steering load than the inner cornering rack. If it is determined the outer cornering rack has a greater steering load than the inner cornering rack, the value of the difference between the outer cornering steering load and the inner cornering steering load is compared with a given threshold value. If it is determined the difference between the outer cornering steering load and the inner cornering steering load is greater than a given threshold value then the outer cornering steering motor and the inner cornering steering motor are mechanically coupled together through the differential to form a mechanically coupled steering motor unit which provides added torque to the outer cornering rack.
Once the difference between the outer cornering steering load and the inner cornering steering load is determined to have fallen below a given threshold value, the outer cornering steering motor and the inner cornering steering motor are uncoupled through the differential. The torque produced by the outer cornering steering motor is then provided to the outer cornering rack and the torque produced by the inner cornering steering motor is provided to the inner cornering rack.
In still another aspect of the present invention, a method of operating a steering system is provided. The method involves detecting a desire by an operator for a rapid deceleration condition. Once a desire for a rapid deceleration condition by an operator has been detected, the first rack and the second rack are mechanically coupled together through the differential. The coupled steering motor unit moves the first steering rack and the second steering rack together in such a fashion to create an increased toe-in condition for the first steerable wheel and the second steerable wheel to assist in slowing the vehicle.
Once a desire by an operator to return to a normal driving condition is detected, the first rack and the second rack are mechanically uncoupled. The first steering motor and the second steering motor are then moved in such a fashion to remove the increased toe-in of the first steerable wheel and the second steerable wheel thereby returning the first steerable wheel and the second steerable wheel to a normal driving position.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.